Plans to ditch Boris Johnson as Tory leader and prime minister “sooner rather than later”, without waiting for a clear and obvious successor to emerge, are being advanced by a growing number of senior Conservatives. Amid mounting alarm at the effects the Partygate scandal could have on their electoral chances, high-ranking MPs are urging wavering colleagues not to dither because of worries about the succession, or Russia’s invasion of Ukraine, but rather to strike before it is too late. One former minister told the Observer: “Things have changed. There is now a feeling that we can’t defend what is going on and that we can’t delay any longer because of the succession or Ukraine. If we don’t act well before the party conference in October, it will be too late.” After another disastrous week for Johnson, during which MPs of all parties agreed to set up their own investigation into whether he deliberately misled parliament, MPs say several potential successors are stepping up campaign preparations and canvassing support – including Liz Truss, Penny Mordaunt and Jeremy Hunt. Last night Mark Harper, the former Tory chief whip, who last week called for Johnson to go, told the Observer he understood why some colleagues wanted to delay acting until after Partygate investigations were complete, or for other reasons. But he urged them to have confidence that an excellent successor would emerge during the process to elect a new leader, even if it was unclear now who that would be. “I have seen enough to reach a conclusion that the prime minister needs to go,” Harper said. “My colleagues can be confident that we have very talented people and a very robust process for selecting a new leader that will ensure we get a capable, credible successor who can set out an attractive proposition and ensure we can win the next election.” He added: “I think the facts will mean that a majority of Conservative MPs will reach the conclusion that the prime minister needs to go.” Another former cabinet minister said doubts over the succession were now irrelevant, such was the urgency of the situation. “A broomstick would be better than what we have at the moment,” he said. The mood swung dramatically against Johnson last week after he was forced to apologise to the Commons for being fined for attending a lockdown birthday party in 2020, and then appeared to back attempts to order his own MPs to block an investigation into whether he had deliberately misled parliament by previously denying parties had taken place. Many Tory MPs are waiting until the results of the 5 May local elections before deciding whether to send a formal letter of no confidence in Johnson to the chair of the backbench 1922 Committee, Sir Graham Brady. If Brady receives 54 or more such letters, a vote of confidence has to be held, and if Johnson loses he must step down. The problems that have recently engulfed the previous favourite to succeed Johnson, the chancellor, Rishi Sunak – over his wife’s tax affairs, and the fact that he, too, was fined for breaking lockdown rules – are seen as having destroyed his chances of following Johnson into No 10. The Tory peer Lord Hayward agreed that the mood had changed and succession issues were less likely to hold MPs back. “I get the impression that because people have moved from whatever position they were in to a more sceptical position, they now believe it’s just got to happen – and we’ll have the conversation about ‘who next’ when it happens. There are people moving, reaching the point of no return, even though they’re not sure who the alternative would be. It’s only a question of judging when the moment arrives.” Another senior Tory MP said: “Lots of colleagues who have local elections going on have been hearing, ‘I like what you’re doing as our new MP, but we can’t vote for you while that blithering idiot is in office’. I spoke to several MPs this week who had all taken their seats from Labour. They all said the situation was terrible. They’re starting to say, it’s not whether but when – and it doesn’t matter who, as long as it’s not him.” Johnson insisted during a two-day trade trip to India that he would lead his party into the next general election. But the Partygate scandal dogged him throughout the visit. Shortly before his return to London on Friday, it emerged that the Metropolitan police had started to issue fines for another party he attended – in this case a “bring your own booze” garden event on 20 May 2020. No 10 said Johnson had not been fined for attending the event, though it was impossible to say whether he would be in the future. Opposition party leaders said it would be unacceptable for the prime minister to hold back information about whether he had been fined ahead of the local elections. Labour’s deputy leader, Angela Rayner, said: “There can be no hiding place for law breaking, and that includes in Downing Street. Boris Johnson must keep his promise and declare without hesitation if he is given another fine for breaking his own lockdown law. He has withheld the truth from the British public for far too long.”